The drilling of an oil or gas well is typically carried out by use of a drill bit connected to the lower end of a drill string. The drill bit penetrates the earth to create the well bore, with the drill string continually lengthened as the drill bit cuts deeper into the earth.
Drilling fluids are required to facilitate the drilling operation. The drilling fluids are circulated through the drill string down to the drill bit and are used to control subsurface pressures and carry away cuttings and other debris from the bottom of the well bore.
The term "underbalanced drilling" or "reduced-pressure drilling" refers to drilling operations involving the circulation of a drilling fluid usually having a density less than that of water. Such drilling fluids include dry gas, mist, foam, stiff foam, aerated water, and aerated drilling mud. For example, gas drilling of oil wells, that is, drilling with the use of air or other gas as the circulating drilling fluid, is well known in the industry. Gas drilling has the advantages of increased drilling bit life and reduced drilling times. Thus, the principal benefit derived from the use of air and aerated drilling fluids is a gain in penetration rate resulting from the negative or at least reduced differential pressure between the drilling fluids and the surrounding earth.
When oxygen,containing gases, such as atmospheric air, are used as the circulation drilling fluids or are used to aerate oil-based drilling mud, there is some danger of a downhole fire and explosion. Industry experience is that downhole fires are rare incidents, however, some such downhole fires and explosions have been reported. It is believed that these have been associated with plugging of the annulus in the well bore. As such, there is a resultant increase in pressure in the system immediately prior to the explosion occurring. The adverse consequences of downhole fires are quite significant. Typically, the bottom drill bit and the drill collars and drill bits are melted away making bit recovery operations impossible. As a result, the well bore must be plugged upstream from the melt down area and sidetracked. This results in a significant loss of very expensive equipment and valuable time, thereby adding significantly to drilling costs.
Another problem encountered with underbalanced drilling using atmospheric air or aerated drilling fluids is an increase in corrosion of the drill string and drill bit. Thus, the high oxygen content in the drilling fluids enhance corrosion of the drill parts.
There have been some attempts made in the prior art to overcome these problems. For example, natural gas from a neighbouring well has been used as the drilling fluid. At present gas prices, this has become a prohibitively expensive method. In addition, the use of natural gas presents certain hazards to surface personnel.
In U.S. Pat. No. 3,301,323, a method and apparatus is disclosed for controlling the properties of the drilling fluid. In particular the oxygen and other reactive materials present in the drilling fluid is reduced and maintained at a desired low level by addition of small quantities of scavenger materials to the drilling fluid. The scavenger materials are selected to react with and neutralize either some or all of the oxygen, carbon dioxide and/or hydrogen sulfide in the drilling fluid.
U.S. Pat. No. 3,286,778 discloses a method for minimizing the occurrence of fires during well drilling with air and other gases. The method requires the addition of inert materials to atmospheric air prior to use as the drilling fluid. Inert materials that can be used include gases such as carbon dioxide, exhaust gases and nitrogen; solids such as powdered soda ash; and liquids, including suspensions or emulsions. Thus, the method of U.S. Pat. No. 3,286,774 requires the addition of inert materials to atmospheric air for use as the drilling fluids.
In U.S. Pat. Nos. 4,136,747 and 4,350,505, a method and apparatus is disclosed for reducing the oxygen content of the drilling fluids. The method disclosed includes bubbling a stream of low pressure exhaust gas through a reservoir of liquid drilling fluid. The nitrogen component of the exhaust gas displaces any dissolved oxygen in the drilling fluid. The drilling fluid is then pumped down into the well bore. The method and device disclosed by Mallory et al does not, therefore, use exhaust gas as a drilling fluid, but rather it is used only to remove or displace any oxygen from the drilling fluids.
A diesel exhaust gas purification system for inert gas drilling is described by Caskey in Geothermal Resources Council, Transactions, Vol. 6, October 1982. The described system, since it is for use in association with geothermal drilling, must eliminate entirely the oxygen content from the compressed exhaust gas stream. By virtue of such geothermal drilling the system must attend to O.sub.2 /NO.sub.x removal, CO.sub.2 and H.sub.2 O removal. The O.sub.2 /NO.sub.x removal is accomplished by use of catalytic converters commonly used in the automotive technology. Such catalytic converters are operated at extremely high temperatures to significantly decrease oxygen content to a level of less than 50 ppm and to convert NO.sub.x into nitrogen. The unit is transportable, however, the cost is extremely high, well in excess of $1,000,000. Such high costs is considered prohibitive for use in underbalanced drilling systems even though it might be a satisfactory cost for geothermal drilling.
Accordingly, it is an object of an aspect of the present invention to provide a method and apparatus for providing a stream of inert gases as a primary component of drilling fluids used in underbalanced drilling of a well bore. It has been surprisingly found that use can be made of the high volumes of substantially inert high temperature gas produced by engines used on the drilling site to drive various components of the drilling rig and the gas compressors.